Optical fiber laser amplifiers are a known technology for producing a coherent output beam of intermediate power. A variety of factors, including Stimulated Brillouin Scattering, four wave mixing, and optical damage, limit the output power of a single fiber amplifier to the range of several hundred watts. A laser of this power output may be useful in a variety of applications, but other applications require higher output power than that available from a single fiber amplifier. Even though the ultimate limit is unclear, it is virtually certain for physical reasons that the performance needed for envisioned defense and heavy industrial applications will not be achieved with a single fiber device.
Higher powered laser systems have been constructed by assembling an array of fiber amplifiers driven by a master oscillator. The output beams from each of the fiber amplifiers are coherently combined in the far field to produce a nominally single output beam. In general, in order for the combined beam to have good beam quality, the individual beams must be substantially parallel and collinear.
A variety of approaches have been used to combine the beams from multiple fiber amplifiers. Coherently combining the outputs of multiple fiber laser amplifiers configured in 2-D arrays is one attractive method for scaling up in power that has been successfully demonstrated over the past several years. In addition to requiring that combined beams be relatively parallel and collinear, coherent beam combination requires the precise control of the relative phases of the individual emitters in the array.
A known technique for combining beams emitted by fiber or solid state amplifiers uses a 2-D array of lenses—a “lenslet array”—to combine the Gaussian-like beams in the far field. It is preferable that all of the energy exiting the array be concentrated into a single beam or lobe. Accordingly, the lenslets are typically precision-manufactured on a single substrate, and are spatially close-packed to minimize the so-called array underfill and the resulting emitted power radiated into array side lobes. The beams from each of the lenslets must be pointed in the same direction to within a small fraction of the beam divergence as well, which is accomplished by careful active alignment and secure bonding in position. These considerations are managed by techniques and processes known in the art.
One additional factor controlling whether substantially all of the energy emitted in combined output beam appears in a single lobe is phase or “piston” error, which occurs when the constituent beamlets differ in optical phase. Piston error arises from various sources, including differences in the lengths of the optical paths of the several beamlets, thermal effects, and the like. Some of these error sources vary significantly and rapidly over time. Thus, is it necessary in coherent arrays to actively adjust the phase of each beamlet to form a diffraction limited beam in the far field comprising substantially a single lobe.
In a known type of phase-controlled coherent array, a reference signal is generated by a Master Oscillator (MO) which emits a narrowband signal that is amplified by the array. The signal is preamplified and split up into multiple beam lines, each of which includes an in-line phase modulator that serves to adjust the phase of that line and compensate fluctuations that arise in the amplifier for a variety of reasons. Each amplifier line also has a preamplifier, isolator, and power amplifier feeding a respective output transport fiber. Each of the output transport fibers is collimated by one of the lenslets in the aforementioned lens array. Each fiber preamplifier and power amplifier is pumped by a laser diode operating at a wavelength that is efficiently absorbed by the lasing dopant in the fiber core. The fiber amplifiers efficiently convert the pumping light from the laser diodes to high brightness output beams that must then be appropriately phased to produce a composite diffraction-limited beam for the entire array.
To measure and control phase of the individual beam lines in the known combined array system, a sample of the reference MO beam is frequency shifted by some amount, typically tens to a few hundred MHz, and used to form a collimated reference wavefront. A small sample of the outgoing array beam is combined through relay optics with the reference wavefront onto a photodetector array comprising N elements, where N is the number of fiber amplifiers being coherently combined. The optics are constructed such that each detector receives a signal from the frequency-shifted reference wavefront and a sample of the signal from only one fiber amplifier. The beat signal is at the heterodyne difference frequency and includes the dynamic phase of the fiber beam line which must be adjusted to a fixed constant value with respect to all the other fiber beams for proper operation. The instantaneous phase is extracted by signal processing algorithms and the in-line phase modulator is activated to drive the net phase to the desired value for array beam forming.
Although the aforementioned technique works successfully in smaller arrays, it requires a separate frequency-shifted reference line and a separate optical detector for each fiber in the array. It is expected that future high power arrays may combine a large number of elements—many tens or possibly a few hundred—and therefore the cost of the conventional phase control system can be unacceptably large. In addition, the weight, complexity, and robustness of the phase control equipment becomes unfavorable as the number of elements becomes large.
Thus, the need exists for a laser system that employs a plurality of fiber laser amplifiers, each producing an intermediate beamlet and all such intermediate beamlets being subsequently combined to form an output beam, which laser system provides improved control of piston error with respect to the intermediate beamlets, thereby maximizing the quality of the output beam and substantially concentrating its energy in a single lobe.